David J. Tyler, Ph.D.

Manchester Metropolitan University
Manchester, UK.

Brown coal or lignite, as it is also known, is a grade of coal intermediate between coal and peat. It is yellowish brown in color with a woody texture (fig1). Brown coal contains more moisture and less energy per kilogram than more mature coals. It also tends to dry and crumble when exposed to air. Brown coal is a second tier fuel that produces significant amounts of smoke. Large deposits are found in the United States, Canada, Germany, and elsewhere, chiefly in Tertiary formations.

German Brown coal localities are found from Koln in the west of Germany right across eastwards. The eastern material is not so profitable to mine, and many pits have closed. Cumulative brown coal production has been 187 million tons to 1996. Somewhere between 80-90% is used in electricity production, and power stations are a normal feature of the skyline in brown coal areas.

A large mine may have a life of 50 years, but it "moves" with time. There are problems of relocating and resettling people, land reclamation, management of river courses and water tables. For example, one pit required the relocation of 8,000 people. In this case, 70% of them wanted to retain their community and it was necessary to move the whole village.

Working pits target coal seams of 50-75 metres thickness. These are located at the central parts of large Tertiary basins. The map of the limits of 25 metre thick coal shows it to be very extensive. Normally, mining goes down to about 300metres depth, but it may occasionally reach 400 metres. The whole area has experienced deformation and NW/SE faulting. There are gentle dips. The faults post-date the coal, but may be contemporaneous with the overlying sands. In western Germany, the Tertiary is up to 800 metres thick. Terrestrial sediments are to the southeast, and marine to the north. There is evidence of marine transgression - causing interfingering.

The largest faults are about 600 metres throw. Most are much smaller than this, creating a stepped structure. The area is regarded as one experiencing ongoing extension of the order of a few mm per year. However, this is overridden by subsidence due to water extraction (about 5 metres of movement in total).

Lithostratigraphical work has been undertaken, utilising the clays for correlation purposes. In the coal layers, there are additional biostratigraphical markers. The coals are mainly Miocene, with some Pliocene.

Special machines have been built to extract the coal and the surrounding sediments. Transportation within the mine is by conveyor belts. The machines can be 220 metres long and 50 metres high. The cutting wheel is about 20 metres diameter.

THE MINE

The Tagebau Hambach pit, to the west of Essen (fig 2), was described by our guide as the "biggest hole in the world in soft rock". On the drive to the pit, we passed a large man-made hill on the western margin, 200m high and covered in trees. This hill is said to contain 1 billion cubic metres of material: the tip resulting from the early years of operating the mine.

Work commenced at the mine in 1978 but coal was not reached until 1984. The initial spend was 6 billion marks. The pit is currently about 6 km wide and 320 metres deep. 1600 people work today in the mine, although a few years ago, the figure was 6,000 people. The decline has come about because of the reduction in the world price of coal. At present rates of extraction, the mine will be workable until 2040. Before then, three villages will have to be relocated, plus a major railway line and a trunk road.

Each 50 metre thick coal was said, by our guide, to have been formed in an accumulating peat bog. Timescales have been estimated at 8 million years, but our guide thought this was too long and preferred a figure of 4 million years. In the deposits generally, over 300 plant species have been identified. Trunks of conifers and deciduous trees are present (not homogeneously) but in this pit they did not exceed 2 metres diameter. They were thus not mature trees. Our guide linked this to the peaty environment and its lack of potential for long term growth. A 3-to-1 compaction was thought to have taken place.

Above this seam were sands (said to be marine) and then another coal. Every thing looked well bedded. The top of the upper coal was uncovered by our guide to reveal burrows. They are interpreted as the burrows of a non-marine mussel (Teredina) which were subsequently preserved as pyritised infills. Our guide first found these burrows in 1992. We were informed that burrows have also been found in wood (as well as the peat) in the top of this coal seam. The sand used to be thought of as marine - but are now reinterpreted as non-marine. We were not able to examine them closely, but it was apparent that cross-bedding was a feature of these 70 metre thick sand deposits. Some clay horizons within the sand are said to contain plant material.

We looked also at the overlying Pliocene sediments. In one locality, our guide drew our attention to fossil leaves incorporated into the sands. In addition, there were numerous examples of wooden branches and some trunks of trees. They were all in a highly fractured state, and none of the trunks had roots.

The uppermost Pliocene horizon is a thick clay, interpreted as a river deposit. A variety of fossil materials have been reported: from molluscs to mammals (including primates).

DISCUSSION

The most interesting questions concern whether this brown coal is in situ (autochthonous) or washed in (allochthonous). Relevant data, not all of which has been introduced above, with possible interpretations, are below. It is necessary to note that the depositional model for the allochthonous coal involves unusual and violent processes, whereas the depositional model for the autochthonous coal involves sustained and cyclical conditions over millions of years.

Criterion

Allochthonous

Autochthonous

Bedded structures

Water deposited

??

Fragmented, not decayed, wood

Catastrophism

??

Lack of rooted trunks

Not in situ

??

Immature trees

Destruction of young ecosystem

Ecosystem not able to sustain large trees

300 plant species

Immature ecosystems?

Changing flora with time

High proportion of peaty matrix

Degradation during transport

??

Relative purity (2-3% mineral content)

Water sorting of materials

No insurges of sediment despite subsidence

Great thickness

Deep sedimentary basin and subsidence

Subsidence and growth in situ

Fossil soils

Alleged soils are rare and may not be soils

In situ growth

Bark stripping

Catastrophism

Decay

Absence of volcanics

If volcanism caused the destruction, it was far away

Mt St Helens is not a good analogy

Interfingering of coal and marine sands

Both are water lain and contemporaneous

??

Siderite occurrences in certain areas

Percolating groundwaters

Percolating groundwaters

Oriented wood in places

Water laid

Not a general pattern - a chance orientation?

Fossil content of sands/muds

Potential environmental relevance

Potential environmental relevance

From this comparison, the ONLY strong argument for in situ growth is that there are seat earths. Despite our strong request to our guide to show us an example, we did not see one. The reason was: shortage of time. A 50 metre thick coal should have numerous horizons within it where renewed growth would have occurred. However, there was no hint of such horizons. I looked carefully for root systems - but found nothing.

The arguments for the coal having been washed in, by violent processes, seems overwhelming. It remains an open question what the mechanisms were for this catastrophic model.